[GlobalISel] Add missing operand update when copy is required

When constraining an operand register using constrainOperandRegClass(),
the function may emit a COPY in case the provided register class does
not match the current operand register class. However, the operand
itself is not updated to make use of the COPY, thereby resulting in
incorrect code. This patch fixes that bug by updating the machine
operand accordingly.

Reviewed By: dsanders

Differential Revision: https://reviews.llvm.org/D91244